The present invention relates to apparatus and method for recording and reproducing digital video and audio data to and from a record medium and, more particularly, to a recording/reproducing device which is operable to receive and transmit video data from and to multiple external devices.
As is known, television broadcasting stations produce television programs by imaging video images to produce video signals (e.g., using a television camera), recording the video signals on magnetic tape using video tape recorders, editing, if necessary, the video signals stored on the magnetic tape, assembling the edited or unedited video signals, and recording the assembled video signals on a magnetic tape for broadcast at a later time. Generally, there is a relatively large amount of unassembled video signals stored on magnetic tape (commonly called "raw footage") and, thus, it is typical that the process of selecting and editing desired "footage" is slow and inefficient. In addition, a large amount of magnetic tape is not easily moved between editing rooms and video signals stored on the same magnetic tape cannot easily be processed (i.e., edited) by different editing devices at the same time.
It is common for a television program, for example, a news program, to be recorded on a magnetic tape and be loaded into a video tape recorder (or a cart machine having a video tape recorder therein) prior to transmitting the program recorded thereon. One difficulty encountered in such an arrangement is the general inability to quickly and efficiently modify the video signal that is about to be transmitted.
Another device, known as a "server system", is used to centrally manage the "raw footage" so that it can be more easily edited, assembled and transmitted. FIG. 1 is a block diagram of a server system 4 which is utilized to transmit and receive video and audio data to and from multiple external devices. As shown, server system 4 is comprised of a central processing unit (CPU) 401, a read only memory (ROM) 402, a random access memory (RAM) 403, a high speed bus 404, plural recording/reproducing devices 406.sub.1 -406.sub.m, various interface devices 405.sub.1 -405.sub.m and 407.sub.1 -407.sub.n, audio/video data compression devices S40.sub.1 -S40.sub.n, and audio/video data decompression devices S43.sub.1 -S43.sub.n. In addition, a LAN interface 411 is provided for server system 4 to interface with a LAN 410 when the server system is not connected directly to the specific application system utilized.
Video and audio data to be recorded on a magnetic tape are supplied from an external device (not shown in FIG. 1) to one of the compression devices 408 which compresses the supplied data in a manner well known in the art and which supplies the compressed data via interface 407 to bus 404. The compressed data on bus 404 is stored temporarily in RAM 403 SO that the rate of data transfer can be controlled and the data stored in RAM 403 is supplied via bus 404 and interface device 405 to one of the recording/reproducing devices 406 which records the compressed video and audio data on a magnetic tape contained therein. During a reproduction operation of server system 4, video and audio data recorded on a magnetic tape is reproduced therefrom in one of the recording/reproducing devices 406 and supplied via interface 405 and bus 404 to RAM 403 which stores temporarily the reproduced data therein. The stored data is output from RAM 403 and supplied via bus 404 and interface 407 to one of the decompression devices 409 which decompresses the reproduced data and which supplies the decompressed data to an external device, for example, a data editing device (not shown in FIG. 1).
In response to instructions supplied via LAN 410 from an application system (not shown), CPU 401 controls the various devices of server system 4 in a manner well known in the art. Server system 4 further may include control lines S44.sub.1 -S44.sub.n which transmit and receive control signals to and from the external devices and CPU 401 controls the operation of server system 4 in accordance with those control signals.
One difficulty encountered in typical server systems is the general inability to increase the number of external devices that can be coupled thereto. That is, server system 4 is not easily expandable beyond a set number of interfaces connected thereto due to the limited bandwidth of its data bus. As is known, digital video/audio signals have a bit rate of at least 100 Mbps (million bits per second) and compression encoded video/audio data have a bit rate of 30 Mbps. If a server system, such as shown in FIG. 1, includes eight editing devices which are used simultaneously to edit (e.g., AB roll edit) two magnetic tapes having raw footage thereon, the raw footage has a bit rate of 30 Mbps (or approximately 4 million bytes per second (MBps)), and the video/audio data is buffered in RAM 403 as discussed above, then 48 "sets" of encoded audio/video data simultaneously are transmitted, at a maximum, on bus 404. That is, each of the 8 editors is responsible for reproducing 2 sets of raw footage and recording (in server system 4) an edited video signal for a total of 3 sets of audio/video data per editor, and each set is stored in RAM 403 and subsequently output therefrom. Thus, 6 sets of audio/video data may be transmitted on bus 404 for each editing device, for a total of 48 sets. If each of the 48 sets of audio/video data has a byte rate of 4 MBps, then bus 404 must have a transmission rate of at least 192 MBps to ensure that 8 editing devices can operate to edit the audio/video signals simultaneously. However, since the data bus of a typical high performance server system has a transmission rate of approximately 100 MBps, it is not possible or, at least, not efficient to utilize 8 editing devices therewith.
Another difficulty encountered in typical server systems is their general inability to ensure that interruptions in transmission of audio/video data to an external device does not occur. This is particularly important when the external device is a broadcast transmission system which is currently transmitting a television program.
A further difficulty encountered in typical server systems is their general inability to communicate in a synchronous manner with television broadcasting equipment that typically operate in synchronization with one another. A server system which utilizes a central processing unit and which has the block structure shown in FIG. 1 operates in an asynchronous manner, whereas it is necessary that the audio/video data that is transmitted between the server system and external broadcasting equipment be in synchronization. This, however, is not easily achieved in the above discussed server systems. Although synchronization may be achieved by synchronizing each of the signals to be output in synchronous/asynchronous converters contained within the server system, providing such a converter for each of these interfaces substantially increases both the size and cost of the server system.
Another problem encountered in the above-discussed service system is that transmissions by bus 404 and RAM 403 cause delays in transfer between the recording/reproducing devices, the interfaces 407 and the external devices. These delays require that data supplied from and transmitted to interface devices 405 and 407 be buffered in RAM 403 during both reproduction and recording operations of the system server thus making it difficult to satisfy any "response performance requirements" of the external devices. Also, the inherent delays of the asynchronous devices of the server system generally vary as a function of both time and temperature and, thus, synchronizing the output of the server system is quite difficult to carry out.
As previously discussed, server system 4 may be controlled by utilizing control signals supplied over control lines S44.sub.1 to S44.sub.n. However, control signals supplied via control lines S44.sub.1 to S44.sub.n to interface devices 407.sub.1 and 407.sub.n must be transmitted via the asynchronous bus 404 to CPU 401 and back over the asynchronous bus 404 to the various devices in the server system and, thus, synchronization cannot simply be achieved by synchronizing the control signals supplied to the server system.